Add SegmentedControlIOS

Summary:
Fixes #534:

![screen shot 2015-03-31 at 7 52 10 pm](https://cloud.githubusercontent.com/assets/153704/6934038/742ddd34-d7e3-11e4-8f55-3eb7d9d3f1cd.png)

```jsx
<SegmentedControlIOS
  tintColor="#ff0000"
  values={['One', 'Two', 'Three', 'Four']}
  selectedtIndex={0}
  momentary={false}
  enabled={true}
  onValueChange={ (value) => console.log(value) } />
```

This only supports string-based segments, not images. Also doesn't support full customization (no separator images etc); I figure this is a good MVP to lock-down a basic API

I also included a snapshot test case, but the images keep coming out funky. When I look at the sim, I see that the text labels show up for the selected segment, but the snapshot keeps coming out with no text on those segments. I tried forcing a delay, but same result. Is that explainable?

Obviously happy to change anything about the API, code-style nitpicks, etc
Closes https://github.com/facebook/react-native/pull/564
Github Author: Clay Allsopp <clay.allsopp@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

React/Views/RCTSegmentedControl.m

@@ -0,0 +1,57 @@
+//
+//  RCTSegmentedControl.m
+//  React
+//
+//  Created by Clay Allsopp on 3/31/15.
+//  Copyright (c) 2015 Facebook. All rights reserved.
+//
+
+#import "RCTSegmentedControl.h"
+
+#import "RCTConvert.h"
+#import "RCTEventDispatcher.h"
+#import "UIView+React.h"
+
+@implementation RCTSegmentedControl
+{
+  RCTEventDispatcher *_eventDispatcher;
+}
+
+- (id)initWithEventDispatcher:(RCTEventDispatcher *)eventDispatcher
+{
+  if ((self = [super initWithFrame:CGRectZero])) {
+    _eventDispatcher = eventDispatcher;
+    _selectedIndex = self.selectedSegmentIndex;
+    [self addTarget:self action:@selector(onChange:)
+   forControlEvents:UIControlEventValueChanged];
+  }
+  return self;
+}
+
+- (void)setValues:(NSArray *)values
+{
+  _values = [values copy];
+  [self removeAllSegments];
+  for (NSString *value in values) {
+    [self insertSegmentWithTitle:value atIndex:self.numberOfSegments animated:NO];
+  }
+  super.selectedSegmentIndex = _selectedIndex;
+}
+
+- (void)setSelectedIndex:(NSInteger)selectedIndex
+{
+  _selectedIndex = selectedIndex;
+  super.selectedSegmentIndex = selectedIndex;
+}
+
+- (void)onChange:(UISegmentedControl *)sender
+{
+  NSDictionary *event = @{
+    @"target": self.reactTag,
+    @"value": [self titleForSegmentAtIndex:sender.selectedSegmentIndex],
+    @"selectedSegmentIndex": @(sender.selectedSegmentIndex)
+  };
+  [_eventDispatcher sendInputEventWithName:@"topChange" body:event];
+}
+
+@end